The present invention relates to flake-like α-alumina particles having a very high aspect ratio which exhibit good dispersibility when being kneaded as fillers or pigments in rubbers or plastics or as coating agents, with a resin and which can be easily dispersed as primary particles in aqueous solvent with high dispersion stability and desirable orientation when added to an aqueous slurry of precision abrasives or cosmetics. The present invention also relates to a method for producing such flake-like α-alumina particles. Further, the present invention relates to a cosmetic containing the flake-like α-alumina particles, and more particularly a cosmetic that has a good tackiness to the skin and a pleasant smoothness in use, covers spots and freckles, and provides a suitable gloss and luster and a transparency that does not darken.
Known methods for producing plate-like alumina particles by using, as a starting raw material, alumina gel obtained by neutralization of aluminum hydroxide or aluminum ions obtained by Bayer's process etc., include a calcination process comprising addition of a mineralizer as disclosed in Japanese Patent Publication No. 35-6977. However, the particles processed by such a known method contain a large proportion of aggregated flaky particles and when added to a resin component, an improved mechanical strength of the resin component or a feeling of gloss can hardly be obtained.
In view of the foregoing, there was proposed a method for producing plate-like particles having a uniform particle shape and good dispersibility by conducting a hydrothermal treatment on an aqueous slurry of aluminum hydroxide in an autoclave and the obtained particles exhibited an improved dispersibility when kneaded with a resin component (Japanese Patent Publication 5-17132A). However, when these plate-like particles are added to an aqueous solvent to be used in a slurry form of precision abrasives or cosmetics, even such improved alumina powder cannot smoothly advance dispersion in the solvent and tends to cause re-aggregation of particles once dispersed in the solvent. As a result, such alumina particles still have problems or difficulties in reducing the surface roughness of an article to be polished or in providing a cosmetic powder with a sufficient spreadability and a capability to develop gloss after application, etc.
In order to give a fine texture and improved mechanical physical properties to molded final products and to improve the stability of a coating film and a slurry as well as the gloss of a dried product after application, flake-like particles are required to have dispersibility in a resin component or various solvents. In order to meet such requirements, the particles have to be insusceptible to aggregation and have affinity to the dispersing medium and the surface state of electrically charged particles have to be in a desirable state.
Especially, when the conventional flake-like particles are used in cosmetics, further problems have been encountered. Pigments that have been added to conventional cosmetics include talc, mica, kaolin, sericite, and other extender pigments, titanium dioxide, zinc oxide, and other white pigments, iron oxide, chromium oxide, ultramarine, Prussian blue, and other coloring pigments, and, if needed, titanium oxide fines and other such ultraviolet shielding pigments or titanium, mica, and other such sheen agents.
An extender pigment has a poor hiding power because its refractive index is close to that of oils used in combination, but is added because it improves the feel of a cosmetic (how well it adheres and spreads and how smooth it makes the skin feel) and enhances moldability. White pigments generally do have hiding power because of their large refractive index, and are added in order to cover up spots, freckles, and the like on the skin when the cosmetic is applied.
Coloring pigments impart a color to the skin, and are added in order to give a healthy and attractive appearance. Ultraviolet shielding agents, sheen agents, and the like are also added as needed.
These various pigments are mixed as dictated by the intended use of the cosmetic, and the cosmetic is produced by suitably adding higher aliphatic alcohols, higher fatty acids, ester oils, paraffin oils, waxes, and other such oil components, ethyl alcohol, propylene glycol, sorbitol, glucose, and other such alcohols, mucopolysaccharides, collagens, lactates, and other such humectants, various surfactants, thickeners, antioxidants, pH buffers, preservatives, perfumes, and so forth.
When various pigments are compounded in a cosmetic, talc, mica, kaolin, sericite, and other such extender pigments are generally in the form of flake particles of silicates that are found in nature. This shape is what provides smoothness, tackiness and spreadability when the cosmetic is applied to the skin, but because these natural products contain coloring components such as iron as impurities, the oils contained in the cosmetic or perspiration from the skin can decrease the transparency of the cosmetic over time, resulting in what is known as “darkening.”
At the same time, another important characteristic that is required of an extender pigment is that it impart gloss or luster that has the look of natural transparency, so that the made-up skin looks fresher. Crushed mica and the like have been used in recent years in an attempt to meet this requirement, but the luster and gloss thereof are greatly affected by the flake-like particle thickness and the smoothness of the particle surface, and with a crushed material, particle smoothness is lost and a luster and gloss having the look of natural transparency are not achieved.
Titanium dioxide, which is the most commonly used white pigment, has a very high refractive index, and therefore it scatters light very well and has good hiding power. However, with a cosmetic containing a large proportion of a pigment with such a high refractive index, the hiding power is actually too high, which can give a feeling of a thick makeup, so that the makeup cannot be finished with a natural feeling. Titanium dioxide also generally tends to aggregate, and has a high coefficient of friction, so another drawback to its use is that smoothness and other such aspects of usage feel deteriorate.